There is a kind of auxiliary lock disclosed by U.S. Pat. No. 4,957,315, which has a moving member 4, a connecting plate 5 and a dead bolt 2 to be extended out of or be retracted in by the connecting plate 5 and the moving plate 4. But pins 41, 61, for connecting same are of round shape and tend to drop off after a certain period of use. Besides, the moving member 4 has two flat sides forming a 90.degree. angle and a plate spring 87 contacting either of these sides to stop rotation of the moving member 4, thus keeping the moving member 4 in one of two positions. This results in friction between the plate spring 87 and the moving member 4 during rotation of the moving member 4, and the plate spring 87 may break off after 20,000-30,000 revolutions of rotation of the moving member 4.